1,3-Butadiene is a conjugated diolefin, largely used as a monomer in the production of synthetic rubber or polymerized with styrene or acrylonitrile. Smaller amounts of butadiene are used to make the solvent sulfolane, nylon via the intermediate adiponitrile, and other synthetic rubber materials such as chloroprene. Butadiene is also used in the industrial production of cyclododecatriene via a trimerization reaction.
The original commercial routes to butadiene were based on alcohol or acetylene and are no longer in use. Existing manufacturing processes all use petroleum feedstocks and are directly linked to petroleum/petrochemical operations.
In the United States, Western Europe, and Japan, butadiene is produced as a byproduct of the steam cracking process used to produce ethylene and other olefins. When mixed with steam and briefly heated to very high temperatures (often over 900° C.), aliphatic hydrocarbons give up hydrogen to produce a complex mixture of unsaturated hydrocarbons, including butadiene. The quantity of butadiene produced depends on the hydrocarbons used as feed. Light feeds, such as ethane, give primarily ethylene when cracked, but heavier feeds favor the formation of heavier olefins, butadiene, and aromatic hydrocarbons.
Typically, there are hydrocarbons in the mixture whose normal volatilities are such that separation cannot be readily achieved by ordinary fractional distillation. For example, butanes are separated with difficulty from butenes and butadiene is likewise difficultly separated from the butenes.
Conventionally, butadiene has been separated from its corresponding olefins and paraffins by an extractive distillation in the presence of a polar solvent, selected for its ability to increase the volatility of some components in the mixture relative to other components in the mixture, with the result that separation of the desired component by distillation is made possible. Polar solvents such as acetonitrile, acetone, furfural, dimethylformamide, dioxane, phenol and N-methylpyrrolidone, and their corresponding aqueous admixtures have been used in extractive distillation processes for butadiene.
Extraction of butadiene from a steam-cracker product generally offers a lower cost material than from a C4 dehydrogenation plant. In conventional extractive distillation processes, the butadiene product is recovered directly from the solvent in a stripping zone at elevated temperatures. The energy required to effect the separations in the extractive distillation zone and in the stripping zone is typically supplied by a reboiler attached to each zone.
U.S. Pat. No. 3,436,436 proposes a process for fractionating a hydrocarbon mixture comprising conjugated diolefin by extractive distillation to obtain a more readily soluble hydrocarbon fraction and a less readily soluble hydrocarbon fraction. An extractive distillation column, a stripping column and a recovery column are employed. The process proposed includes the steps of passing the bottom liquid of the extractive distillation column into the recovery tower, returning the recovered gas from the recovery tower to the extractive distillation column by means of a compressor and introducing the bottom liquid of the recovery tower into the stripping column to recover the extracted materials, whereby the extractive distillation is carried out without raising the bottom temperature of the extractive distillation column, so that the diolefin and higher acetylene may be prevented from polymerization.
U.S. Pat. No. 3,436,438 proposes a process for the separation of a conjugated diolefin from a C4 or C5 hydrocarbon mixture containing the diolefin and higher acetylenes. According to the proposed process, the hydrocarbon mixture is subjected to extractive distillation with a solvent that comprises an N-alkyl-substituted lower aliphatic acid amide. By means of the extractive distillation, the diolefin is recovered in the form of a distillate that is substantially free from higher acetylenes. A liquid extract containing the higher acetylenes and the solvent is also recovered, the extract subjected to stripping to recover the acetylenes and the solvent.
U.S. Pat. No. 3,798,132 proposes a process for separating butadiene from mixtures of C4 unsaturates that includes introducing the butadiene-containing mixture into an extractive distillation zone wherein it is distilled in the presence of a selective polar solvent, introducing the fat solvent to a first stripping zone operated at a pressure lower than that of the extractive distillation zone wherein the butadiene is stripped from the fat solvent thereby forming a butadiene-rich vapor phase, compressing the butadiene-rich vapor phase to a pressure higher than that of the extractive distillation zone, returning a portion of the compressed vapor to the bottom of the extractive distillation zone and introducing the remaining portion of the compressed vapor to a second stripping zone wherein butadiene is recovered as an overhead product.
G.B. Patent No. 2,077,753 proposes a process for the extractive distillation of hydrocarbons. The process proposed includes the steps of extractively distilling a hydrocarbon mixture in an extractive distillation apparatus using a polar extractive solvent. A first-stage extractive distillation column is operated at a higher bottom pressure than the bottom pressure of a second-stage extractive distillation column and a pre-stripping column is provided between the first-stage extractive distillation column and a first-stage stripping column and operated at a pressure equal to or lower than the bottom pressure of the first-stage extractive distillation column and equal to or higher than the bottom pressure of the second-stage extractive distillation column.
W.O. Publication No. 2005/037396 proposes a process wherein a butadiene-rich stream is transferred to a flash drum and the initial expansion causes separation of vapor from liquid, with the separated vapors flowing to a distillation column. Overhead solvent-free concentrated butadiene vapors are obtained and condensed in a water condenser and the first butadiene stream and the liquid stream flows to the stripping column to forming an overhead concentrated butadiene vapor. After compression, a second butadiene stream is obtained.
Despite these advances in the art, there is still a need for an improved process for the separation of a conjugated diolefin from a C4- or C5-hydrocarbon mixture.